It's because it is complex. And in the long run, things become simpler. The only difficulty is the initial setup and once you are past that, the overall maintenance workload just becomes easier compared to a single VM setup
You can even think of it as paying insurance premiums upfront. You get to "make a claim" if the requirements do grow into the sort of need that suit such a cluster/complex setup.
But, on the same insurance theme; I am not sure paying 10K a year to insure my 5K car makes a lot of sense, because, in the long run, I might write my car off.
aka, you're front loading the complexity.
You can even think of it as paying insurance premiums upfront. You get to "make a claim" if the requirements do grow into the sort of need that suit such a cluster/complex setup.